You can ask your ophthalmic practitioner or optometrist for an accurate PD measurement after your eye exam. Usually, opticians do not give out your PD measurement as the law does not require them to give it to you. Some opticians may even charge you for dispensing your PD information.
Your dispensing optician should ensure that your glasses fit correctly and that the pupillary distance on your lenses is correct.
If you don't know your pupillary distance, you can measure it yourself at home with the help of a friend or the mirror.